In the past the DC signal input voltage to an electronic measuring and monitoring instrument such as a cathode ray oscilloscope for use with both AC and DC input signals has been limited to the breakdown voltage of the input probe cable and/or the breakdown voltage of the AC/DC input selector switch. In such instruments a series connected capacitor couples the probe cable to the AC input terminal of the AC/DC selector switch, and if a DC signal is applied to the probe while the selector switch is in the AC position, the capacitor charges to the DC signal voltage and applies that voltage between the AC and DC input terminals of the switch. While the capacitor could be designed to have a high breakdown voltage, and a large, high voltage selector switch could be employed to withstand the higher DC input voltage, such a solution would not only be costly but would result in increased loading for high frequency input signals because of the increased stray capacitance inherent in bulky high voltage elements. Similarly, the probe cable could be made larger so as to withstand higher DC input voltages, but that would also be very costly.